Modulated detector reception of continuous waves



July l1, 1950 mfl A. "G'lFFlvN ET`AL MODULATED DTECT'OR RECEPTION 0F CONTINUOUS WAVES ZVSheets-Sheet 1 NNN 4 www.

BVCQ i ATTIORNEY July 11, 1950 f l n A G RlFFlN ErAL 2,514,859

MODULATED-DETECTOR RECEPTION CONTINUOUS WAVES l 2 Sheet-Sheet 2 Flec'l June ll-. .1946

ATTORNEY f vf'UNiTED STATE Patented july 11,;.1QS

CONTINUOUS WAVES 'Dam G'rifn, North Plainfield, N.":r., and

Leland' C. `Waller, Bayside, N; Y ACoinmiu'iication 1 Measurements as'sign'ors to Laboratory,

New York, N. Y., a partnership 'con'sistingl'nf Dana A. Griffin, Brunson S.

McCutchen, and

William J. Hammond l Application June 11, 1946,'seria1No. 675,854

claims.

fsystem for receiving continuous wave keyed sig- -nals for pulsed signals, such 'as those trans'- mittedby radio telegraph transmitters, pulse -trans`mitters,'and allied types of equipment. i vrillccording to present practice, the usual meth- -od of receiving :keyed .continuous wave l signals employs -a beat `frequency radio frequency oscillator which is tuned to withinabout 1,'000 cycles per second `of the frequency of the last inter- Line'diate frequency signal. l This beat frequency Lis 'then mixedwith` the vsignal in .thesecond detector circuit, producing an audio signal of about x1,000 Icycles per second, corresponding to .the frequencydiiference between the locajl Aoscillator and theincofnin'gsig'zal. t y, @In spite of its v,common use, reception of signals inv this way Vhas a number of disadvantages,

among the most serious of which maybe men-l tioned .the following:

Noisecomponents present in the circuits where ,themiXing takes 4place falso produce beats with ,the ,output of 'the beatlfrequency oscillator, and

this ,is evidenced `by a jump in the noise .level when the beatffrequenriy oscillator is turned on. in addition to the Aiirnage trouble caused by the first frequency converter.' in the case of a superheterodyne, there is an audio image created inthe output. of the second detector, which ocfcurs ,whenever an yinterfering radio frequency signal comes into the second detector, the .-frequency of which happens to be' twice the audio beat frequency away from ythedesired radio frei quency signal, and whose intermediate frequency component is on the opposite side of the beat frequency oscillator frequency from that of the `I. F. vcomponent ofthe desired signal.

This is analogous to image interference, well known in broadcast receivers of the superheterodyne type. This effect can be minimized by the provision of a highly selective intermediate frequency amplifying system, but is rarely entirely eliminated vin actual practice.

-It is yan objectfof our invention to provide a .newsystem andmeth'od for the 4reception of such isignals; Awhich not` only'feliminates -or greatly `reduces many of :the disadvantages y'now Iinherent in thefusuale system,` {but-also .has tmany advantages foverwttie :best of the `presently-.known systems. lMoreover, specifically,

(Cl. Z50- 8) l It is an object offoui" invention to provide a new system of reception of continuous' waveI signals which'will give anlimprovement in .signal` to-noise ratio many times grea'ter than 'can .fb'e

5 obtained from known systems.

AIt is a further `objfect of *our invention to 'Tprovide such a'syst'em which will deli-veraudible signals, the tonelfrequencyof l'vihich remains constant, thus n-iakinfg'fity practicable to. make use of a low tone, heretoforcconsidered impractical because of the diffcultyl'of maintaining the beat note constant. v

It is a furtherobject of'our invention to provide a system which will allow the operatoria choice'o'f tone frequency for received ysignals to reduce operator :aural fatigue.

It is a further object of orurfinventonto pr'ovide a system giving automatic noise .reduction on large noise peaks and eliminating all noise having :sustained small peak values.

`Itis a further object of our invention to provitle automatic audio signal limiting atalmost any desired output voltage levelI regardless of the 'value of inputR; F. signal, over a large input 28 lsignal `voltage range.

It is a further object of our invention to pro- 'vde fa system which gives ineffect the same results' as A. V. C. action in broadcastreceivers,

heretofore generally ineffective when appliedrt 30 continuous Wave .keyed signals. y

` f It is a further object of this invention to pro'- vide a system which effectively increases y the Ioverall gain of .the receiver by jdelivering the .maximum audio output in the sharpest position fof the'lcrystal -lter :selectivity switch, heretofore commonly used, which alsocorresponds tothe maximum intermediate frequency gain, and which Ais 'not true of the beat vfrequency oscillator method of reception heretofore used.

4o t is 'a :further 'object of our invention kto .pro-

vide "atrueffsingle signal method of reception by "medulatingthe signal inr-the audio detector :circuit after the pure I. Ffs'ignal has passed through the selective I. F. .'system, .and without recourse to a local beat oscillator.

is a .further object of this invention topro'- vide improved adjacent *'channel selectivity L"of thereceiver and to eliminate certain difliculties encountered with adjacent -channel interference y 5,0 in receivers of the conventional type.

Cathode 23e of diode 23 may be connected through a lower gate biasing battery 24 and through resistance 25 shunted by condenser 26 to the opposite common terminals of inductance and condenser 22. Anode 21a of diode 21 may be connected through resistance 30 shunted by condenser 28 and through biasing battery 29 to cathode '23e of diode 23 and to ground. Both biasing batteries 24 and 29 may be termed negative delay biases, and their action will best be understood by reference to Fig. 2 and the subsequent portion of this specification, wherein the circuit action is explained.

Fig. 2 is a series of curves illustrating the operation of our invention, and

alternative forms of our invention.

In the various figures, like indicate like parts. In Figs. 3 and 4, for purposes of simplicity, the

parts lying to the left of dotted line `A--A have been omitted, but it will be understood that in setting up the apparatus the parts to the right of lines A-A in Figs. 3 and 4 may be substituted in Fig. 1 for the parts to the right of line A-A. Because, as already stated, Fig. 1 is, for most `purposes, the preferred and simplest embodiment of our invention, its constructionand operation will first be described, afterwhich the `alternative forms in Figs. 3 and 4 will be described. Referring now to Fig. 1, I0 represents an antenna of Yany suitable type, which may be connected to ground through coupling coil I I, which is in turn coupled to input coil I2, tuned by vari- I3. This circuit is connected to the input of the radio frequency amplifier and converter unit I4, `which may be supplied by local oscillator I5, in .turn tuned by inductance I6 and variable conl5. Figs. 3 and 4 are partial circuit diagrams of reference numerals y "A pair of terminals 32 and 33 may be provided, 32 being connected through blocking condenser 3I to anode 21a: of diode 27, and terminal 33 being connected to ground. Between these rterminals IWe connect a suitable source of audio Itone vo ltage` of the frequency which it is desired for the signals to have in the receivers, loud speaker, or other output device, and this source -may-be controllable in. frequency, if desired, to

fable condenser or other tuning instrumentality denser I1, or other tuninginstrumentality,.which latter may be ganged for tuning with condenser I3, asis customary.

A.Since the details of the radio frequency amplifier and converter unit and the local oscillator are well known in the art and are not per se apart of our invention except .to the extent described hereinafter, they are not shown nor described in detail.

The output of converter unit III may be supplied to I. F. amplifier I8 of any suitable type, :gg

and theoutput ofthe I. F.,amplifier may be supplied through coupling coils I9 and 20 tuned by .condensers 2| and 22 to our improved detector circuit. It is our improved detector circuit and the improved methods of detection employedA therein which give our system its advantages as we now understand them.

We employ a pair of rectifiers of similar char- ,acteristicsv While herein shown, for simplicity,

as diodes, they may be triodes or other thermionic rectiers, or may even be crystal detectors, subject only to the condition they shall have, as nearly, as possible, identical or similar characteristics.

In Athe embodiment shown, diode 23 may have cathode 23e and anode 23a, and the anode may be connected directly to one common point of inductance 2D and condenser 22. The other diode 21 may comprise cathode 21o and anode 21a, and its cathode Z'Ic may be connected directly to anode 23a of diode 23. As here shown, the diodes 23 and 21 are in separateenvelopes, but it will be understood that they may both be mounted within a single envelope, if desired.

plitude.

A connection may be made to the common ,pointof resistor 25, condenser 26, and condenser 22, and audio voltage may be withdrawn from this .point andsuppled to a suitable audio amplier and indicatorl not shown, but merely indicated as .the ,arrowhead designated Ewan. From this point, 4a connection may be taken back vto the radio frequency amplifier, converter, and I. F. amplierthrough resistance 35 and lter condenser 36 to` ground, this connection affording `a, voltage which maybe impressed upon the grids of the R. F. amplifier, converter, and I. F. amplifier tubes, whenthe circuit is to be used for conventional A.. V. C. operation on radio-tele- 1Our invention is termed (Modulated Detector Reception. or M. D. R. l

ception of keyed continuous-wave signals, this 'AL V. C". bias is usuallyshorted to ground by switch 3l.

The operation of the receiver will now be explained withreference to Fig. 2, wherein incoming signals impressed on the vdetector circuit are illustrated as Erf. In this figure the curves are plotted as voltage curves about a horizontal zero axis, the upward excursions representing positive and the downward negative voltages, as usual.

The steady voltage of Edd (diode 23 delay bias) is the Vvoltage of bias battery 24, and is equal to Edd (detector thresholdV level or lower gate). In addition, the following relations are established:

E'FEsEM The magnitude of voltageEdd, which operates as a delay bias voltage (I. F. signals must exceed this amplitude before rectication can occur), is so chosen as to eliminate most, if not all, of thev low 'residual background noise so as to insure 'quiet operation at low values of signal voltage, and equals the detector threshold'level, or

lower gatef End.- 'The/:peak magnitude of greater` in .the negative sense, than the maximum signal voltage., and preferably.- EM/is given a square Wave gform ci not too `rapidi rise i andldecay time. With these explanations ofthe voltagesfinvolved, their magnitudes, and vtheir interrelation., the circuit `operation 'may now vbe understood.

Itgii/ill*` be seen thatfdiode 23 `obtains its input signal from theoutputof the I. |F. vampliiier and is so connectedthat itrec'ties vthe positive .half cycles ofthe E. 'signahas `slfro'vvn'in the positivezone of Eig. 2.

` The flower gateV .action .maynowv be uunderstood asl/.it operates -in .the positivej.half. cycle rectification. `Because,or the'lpolarityof battery 24,.,anode 23a is biasednegative with respect to sans cathodegza by the'. amcdutnad, .and-diedero .does 4 not begin to rectifyuntil incoming signals are suiiicient to overcome -voltage -Edd and make diode anode 423c=positive Withj respect.- tofits cathode'. Thisueliminates all rectification'orpositive voltage signals or `noise vuntil they reachthe threshold g Turning now to theaction of diode v21, this .we term the squelching diode, and it will be noted that inv this :rectier a rather more complicated action takesplace. AIn theabsenjce of any voltage provided by ;.source=`34, anode 21a of diode 21 Wouldbe negative with respect to cathode A21c, by voltage Esa, because of the provisionrofbiasing battery 29. .n

` When'source 34isinoperation, the potential of anode 21d swings alternately in a positiveand negative direction, becoming during one-half cycle of source 34, lnegative with referenceto its cathode-only byt-he amount Edd (EsdeEM) and then negative Iwith respectto` its :cathode by the amount Endet-EM.. Dur-ing the'former half cycle,

dio/de Z'lfwill rectify signals more .negative than Eds, whereasduring the `latter vit willnot.

Theresult of thisis rectifiedcurrent foreach positive half of the I. F. signal cyclek above the lower gate valve Edd, as 4shown in the positive.

zone of Fig. 2, and rectified currenty from diode 2l `for eachnegativelhalf o f the I. `signal for each positive half cycle of tonegenerator 34, as shown in the negative zone of .'Fig.'2.

Since these rectified currents flow in opposite directions, duringthe `hal-f cycle. cfthetone generator when vboth ldiodes arerectify-ingI the net result'is zerofrectication duringgeach alternate half cycle ofthe tone generator.` Whendiode r2"! is not rectifying, the netV result is the flow of rectified current from diode 23. ,l

Amplitude limiting on noise lexceeding theupper gate level is provided, because when the 1. F. voltage exceeds the upper gate v`vlevel, both diodes rectify, `and to the V,extent that positive and negative halves of the` I. F. signal wave are symmetrical, the rectified currentoflboth diodes isequal yand, the net current iszero. Because noise is limited both at upper andlowervalues, we term this action .,fidouble `gate `action. .If .battery 24 is eliminated, the flower gate action is alsoeliminated, randi-only upper gate action occurs. f

- Anfad-vantage of `this'circuit is that .Ede can be varied without fchangingwEsdl." For example, if We make-,Edd zero, eliminating they lower gate action, lwe Yneed'on'ly increase by the same amount we -decrease Edd, inorder tomaintain A100% modula- .tion `of 'the signal-` f `It is thefnet rectiedcurrentwhichlgiyes rise voltage, and `it will be-noted that the "flowergate level is established by selecting thevalue of Edd sothatthefresidual ory background or low signal noise level is substantiallyeliminated; .This featurelin'part faceountsior the lack of noise with curr-reoeiverin the absenceof asignal. Y

. YInthe case of av weaksignal, Edd is made as smallas is consistentwith the residual noise level. Thus, when the I. components due to thea-weak signal appear, ythey are modulated in vamplitude -by the I. vF. components due to the noise. Because .the noisey is not usually prominent in the absence of the signal, it can be said that .they tend to .reinforce eachother, but they both are modulated at 4thefchosen audio `freel quency of` tone generator 34.

If this is 800 cycles, a burst lof noise, after the output signal voltageEjpasses through a highly selective A-. lter, whose pass-band includes the frequency of tone generator 34, appears in the telephones `or loudspeaker as a burst of 800 cycles. However., practically .allof the rasping, grating,leand other'unpleasanhsounds usually resulting from -noise in conventionall receivers -have by thisutime been eliminated.

...Additional audio frequency selectivity -can be employed if'desired, by means of -a sharply reso nant .mechanical or piezoelectric reproducer. Thus, it will be seen that our invention permits the fullest utilization'of all three types of receiver selectivity; that is, R. -F., -audio frequency, and mechanical. If thef'average operators ear has la response peak ,at 1;,'000 'cycles per second and the frequency of generator 34 is set at 1,000 cycles. then -a fourth'type of selectivity, thatinherent in the operators ear, can be utilized.

Turning now to the alternative circuits of Figs. 3 and 4-, Fig. 3 differs'from Fig. l principally in the provision of `tube 39,'herein shown as a triode, interposed betweentone generator 34 and the detector circuit, and also in the omission oi the lower noise level bias =battery 24. In .this instance tube 39 may be .a triode containing cathode 39c,contro1-electrode 39g, and anode 39a, and'icontrol electrode-39g may be connected Yto cathode -39c through-leak resistor 40.

Controlelectride 39g may be :connected through condenser 3l to terminal 32 as-before,and anode 39a to a +B source, not shown, the negative terminal of which maybe grounded. Cathode 39e may be connected to 'cathode 21o of diodel 2l, the other connections remaining as before, with the'exception ofthe omission of bias batteries `24-and 29.V Infthis instance theupper gate action .is due'atleastin part to fthe operation of tube'39,

. but no lower gate factionoccurs.

rvcnthelpositive half cycles due to the grid leak actionof resistor 40, and the grid. and control lelectrode 30g rerh'ains 'substantially at Zero bias. ''n the 'negative half cycles, however, :tube 39 reaches plate current cutoff (assuming that 'the gridvoltage from source 34 is suiiicieiitly large),

l 1. and puedeiii'thenreachespracticallyfrom,rect; to both the audio and automatic volume control iicati'on, cancelling 'the current 'utputiof diode 7 23. t will be seen that Whenever the voltagevof source 34 is either zero or positive, diode 24 is not rectifying and suaioreaches a positive maximum. Referring now to Fig. 4, the principal diierence between'Fig. 3 and Fig. 4 is that diodes 23 and 21 are connected reversely with-respect to their direction of current ow. In Fig. 4, therefore,it is necessary to connect resistance 31 in the plate circuit of tube 39 so that the plate current resistance drop across resistance 31 will have its negative polarity facing the plate of diode 21. This arrangement is necessary so that when the voltage of source 34 is zero or positive, triode plate current flows in-tube`39, producing a voltage drop through resistance 31, Whichin turn is the upper gate bias for diode 21.1v v

On the negative half cycles-of source 34, the plate vcurrent of tube 39 is cut' oi so that thevoltage drop through resistance 31 is zero and diode 21 rectifies practically 100% and neutralizes the rectified current of diode 3.` I

As an example of the advantages of our invention, We have found signal/noise ratios of from 150/1 to 200/1 in tests made -with about 0.6 microvolt input at 4.6 meg'aoycles, using a standard SX-ZS I -Iallicrafter feeding our detector system, Whereas under the same conditions, but using the conventional beat frequency oscillator method, about8/l was the best that could be obtained.

Adjacent channel selectivity is also effectively greatly increased. For example, with our system, tWoR. F'. signals only 30 or 40 cycles apart can be completely separated and yet each one can be modulated, in the detector circuit, by a tone frequency of any desired value. The I. F. passband need be only wide enough to pass the A. C. signal components which are a lfunction of the keying speed. Critical adjustment ofthe undesired signal into the zero-beat notch,- such as is necessary with the beat frequency oscillator method, is eliminated in our system. v

We have found that our system provides automatic audio signal limiting at almost any desired output voltageflevel, regardless of the value of input R. F. signal, over a large input signal voltage range. For example, tests show that a variation of input signal' in the ratio of 100,000/1 produces substantially no change in the A. F. output signal level. Y

While We have used the term continuouswave keyed signals in the claims, `we include Within the meaning of such term not only continuous nii-.1423.59

waves keyed manually, but also any form of transmission in which the'transmitter is always full on, or full off, such as in automatic high speed mechanical or electrical keying, as in ypulsed signals.

In the specification We have explained the principles of our invention and the best mode in which we have contemplated applying those principles, so as to distinguish our invention from other inventions; and We have particularly pointed out and distinctly claimed the part, improvement, or combination which we claim as our invention or discovery.

While we have shown and described certain preferred embodiments of our invention, it Will be understood that modifications and changes may be made Without departing from the spirit and scope thereof, as will be clear to those skilled in the art.

We claim:

1. In a receiver for receiving continuous Wave keyed signals, in combination, a pair of rectiiiers having dissimilar electrodes connected together, means for impressing said signals on said rectiers, means for impressing on one only of said rectiers a polarizing voltage of audio frequency producing a periodic variation of rectifier current, means for additionally impressing on said recti- 'ers* respectively *direct 4current delay biasing voltages ofdiierentmagnitudes' and means for combining thecurrent output of said rectiiiers.

2. In a receiver for receiving continuous wave keyed signals, in combination, a pair of ther- Inionic rectiers having dissimiliar electrodes connected together, means for impressing said signals on said rectifiers, means for impressing on one only of said rectilers a polarizing voltage of audio frequency producing a periodic variation of rectier current, means for additionally impressing on said rectiers respectively direct current delay biasing `voltages of different magnitudes and means for"combining the current output of said rectiers.

3. In a receiver for receiving continuous wave keyedr signals, in combination, a pair of diode thermionic rectiers having dissimilar electrodes connected together, means for impressing said signals on said rectiers, means for impressing on oneonly of said rectiiiers a polarizing voltage of audio frequency producing a periodic variation of r'ectier current, means for additionally impressing on said rectifiers respectively direct current delay biasing voltages of different magnitudes and means for combining the current output of said rectiers.

Li. The combination claimed in claim 1, in which the direct current delay biasing voltage on the rectifier having the audio frequency polarizing voltage is greater than the delay biasing voltage impressed on the other rectier.

5, A detector system for continuous wave keyed signals, comprising, in combination, a pair of rectiers having their dissimilar electrodes connected together, means for impressing signals to be detected on saidrectifiers, means for impressing direct current delay biasing voltages of different values on said rectiiiers respectively, means for impressing on the rectifier having the larger delay bias anfalternating polarizing voltage of audio frequency producing a periodic variation in the amplitude of rectified current, and means for combining the current output of said rectiers. l

6. A detector system for continuous Wave keyed signals, comprising, in combination, a pair of thermionic rectiiiers having their dissimilar electrodes connected'together, means for impressing signals to be detected on said rectiers, means for impressing direct current delay biasing voltages of diiferentl magnitudes on ,said rectiers respectively, and means for impressing on the rectier having the larger delay bias an alternating polarizing voltage of audio frequency insufncient at its positive peak to overcome all of the delay bias producing a periodicvariation in the amplitudeof rectified current, and means for combining the'current output of said rectifiers.

7. A detector system for continuous Wave keyed signals, comprising, in combination, a pair of diodel thermionic rectiers having their dissimilar'eleotrodes connected together, means for impressing signals to be detected on said rectiers, means for impressing direct current delay biasing voltages of diierent magnitudes on said rectiers respectively; and lmeans for impressing` on the rectifier having the larger delay bias an alternating polarizingvoltage lof audio frequency the positive peak of which reduces the effective bias of said rectier to substantially the magnitude of the delay bias on the other of said rectiers, producing a periodic Variation in the amplitude of rectified current, and means for combining the current output of said rectiers.

8. The combination claimed in claim l, with means for deriving an automatic Volumeccntrol Voltage from the combined current output of said rectiers.

9. The combination Vclaimed in claim 1, with means for deriving an automatic` volume control Voltage from the combined current output of said rectiiers, and means for impressing said voltage on at least one of the tubes preceding said vrectiers. f

10. The method of demodulating continuous wave keyed signals, which comprises the steps of separately rectifying positive and negative half cycles of signal waves in excess of a predetermined threshold amplitude, periodically preventing the rectication of half cycles of one sign at an audible rate, and combining the rectified currents produced from 'both half cycles in an opposing sense.

DANA A. GRIFFIN, LELAND C. WALLER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,615,636 Langmuir Jan. 25, 192';l 2,194,499 Gelzer Mar. 26, 1940 2,283,404 Wood May 19, 1942 2,293,501 Hansell Aug. 18, 194i?v 2,419,548 Grieg Apr. 29, 1947 FOREIGN PATENTS Number Country Date 192.429 Great Britain Feb. 8. 1923 

